ATI 5770 - can get about 233 MH/s - you can find them for $100.00 or under - they use 100 watts at full
Radeon 5770 offers the best low cost investment option for bitcoin mining. It comes cheap, delivers a punch and isn’t really power hungry – at a little over 100 watts at full load, PSU upgrade is not needed even if you are using cheaper power supply unit
source - http://www.smpake.com/?p=59
Oh, and i just bought one, for $94.49 - that is with shipping
Some stats: if you buy 4 at $100 = $400 .. your 800 MH/s can bring in ฿43.11 ($1,273.17) per month
with NEXT WEEK increased difficulty .. that is still - ฿33.95 ($1,002.49) per month ....
wish i had some money for some more
donations please .. of course i will give you kickbacks later